ACTINOPTERYGII. 81 



ORDER 2. ACTINOPTERVGII. - ^ 



The genus Cheirolepis, which represents the earliest group 

 of Teleostomatous fishes with abbreviate or non-lobate paired 

 fins, is contemporaneous with the oldest Crossopterygians 

 hitherto discovered. Like its numerous allies of a later period, 

 it differs from the Crossopterygians not only in its abbreviate 

 paired fins, but also in its completely heterocercal tail and in 

 the development of a paired series of transversely-elongated 

 rays to replace the pair of gular plates in the branchiostegal 

 membrane between the mandibular rami. Nevertheless, the 

 sub-order it represents agrees with the Crossopterygii in the 

 feebleness of ossification in the notochordal sheath, in the 

 presence of infraclavicular plates, and in the fact that the 

 supports of the dorsal and anal fins are less numerous than the 

 dermal rays apposed to them. 



So far as known, this was the only type of Actinopterygian 

 existing until the Permian period, when another sub-order arose 

 through four simultaneous modifications in the skeleton: 

 (i.) the atrophy of the upper caudal lobe, (ii.) the loss of the 

 baseosts in the pelvic fins, (iii.) the correlation in number of 

 the dermal rays in the dorsal and anal fins with their endo- 

 skeletal supports, and (iv.) the loss of the infraclavicles. In 

 this dominant group of the Mesozoic period, many of the same 

 minor modifications as those of the Palaeozoic sub-order may be 

 observed. 



Early in the Jurassic period a still more specialized type 

 began to appear, when (i.) the mandibular ramus almost or 

 completely lost all elements but two, (ii.) the baseosts of the 

 pectoral fins were reduced to four or five, (iii.) the vertebral 

 centra were always complete, and (iv.) the superficial ganoine 

 gradually disappeared. All members of the latter group, 

 except a few of the earliest, fall under the common de- 

 nomination of "Teleostei," but the fossils afford little or no 

 information as to the characters of the intestine and nonr 

 as to the nature of the optic nerves and heart of the extinct 

 forms. 



\v. 6 



